Where Planets Become Brown Dwarfs: Tracing a Hidden Boundary in the Metal Content of Stars

Giacalone et al. investigate a long-standing question in exoplanet science: when does a giant planet stop forming like a planet and start forming like a star? Although giant planets and brown dwarfs overlap in size, their formation pathways differ. Giant planets are expected to form bottom-up, starting with a solid core that gathers gas, a process that works best in metal-rich environments. Brown dwarfs, on the other hand, form top-down, through direct gas collapse, similar to young stars, and this process depends much less on metal content. The authors take advantage of this distinction by studying how the metallicity of host stars varies with companion mass for objects orbiting between 1 and 50 astronomical units (au). Their approach uses the large, uniform sample of the California Legacy Survey (CLS), enabling them to probe formation physics at intermediate orbital distances where little previous work has been done.

Sample Selection

The authors begin by assembling a carefully selected sample of 85 companions with masses between 0.5 and 110 Jupiter masses (MJup). Because the CLS provides precise stellar metallicities and well-characterized orbits, the team can cleanly examine whether stars hosting the most massive companions differ chemically from those hosting lower-mass giant planets. They also incorporate improved companion-mass estimates that combine radial-velocity measurements with astrometric data from Gaia, allowing them to model the true masses rather than relying on minimum masses. The resulting dataset maps out a clear view of host-star metallicity across a wide range of companion masses.

Analysis and Transition Mass

To determine whether there is a distinct break point in stellar metallicity as companion mass increases, Giacalone et al. employ a hierarchical Bayesian model. This statistical framework tests whether the sample is best described by one continuous population of metallicities or two populations separated by a transition mass. Their analysis strongly supports a change-point model, revealing a transition at 27 MJup. Below this threshold, stars tend to be metal-rich, with an average metallicity around [Fe/H] ≈ +0.17 dex. Above the threshold, stars hosting more massive companions have metallicities consistent with near-solar or slightly sub-solar values, around [Fe/H] ≈ −0.03 dex. The study also finds that the likelihood of this transition occurring below 10 MJup, as some earlier studies suggested, is less than 1%, pointing to a significantly higher boundary between planet-like and star-like formation at these orbital distances.

Orbital Eccentricity Trends

Beyond metallicity, the authors explore whether orbital eccentricity supports a similar division. Objects below the transition mass are more likely to occupy low-eccentricity orbits, consistent with the gentle shaping provided by protoplanetary disks in bottom-up formation. In contrast, higher-mass companions show higher average eccentricities, in line with expectations for top-down collapse and dynamical interactions. Although the shift in eccentricity is not as sharp as the metallicity transition, the two trends together strengthen the case for distinct populations.

Comparison to Previous Work

Giacalone et al. compare their results to earlier studies using atmospheric composition, orbital behaviors, and occurrence rates. Many of these works hinted that the transition between formation mechanisms lies somewhere between 10 and 30 MJup, depending on the orbital distance. Their result of ~27 MJup at 1–50 au fits neatly within this broader picture and aligns with theoretical expectations: forming objects much above ~20 MJup through core accretion requires unusually massive and long-lived disks, conditions that are thought to be rare. Thus, the metallicity transition uncovered here likely marks the upper limit of efficient bottom-up formation in typical planetary systems.

Conclusion

In summary, this paper demonstrates that stars hosting giant planets and brown dwarfs beyond 1 au exhibit two distinct metallicity populations, separated at roughly 27 MJup. This transition mass provides a powerful observational guide for distinguishing planet-like and star-like formation, and it sets the stage for interpreting the thousands of new substellar companions expected from future surveys such as Gaia DR4 and upcoming direct-imaging missions.

Source: Giacalone